To determine the composition of the exhaust gas of an internal combustion engine, i.e. to obtain information concerning the ratio of the air/fuel mixture supplied to the cylinders, oxygen sensors and/or nitric oxide or hydrocarbon sensors are used along the engine exhaust pipe, up- and/or downstream from the catalytic converter.
All currently marketed sensors, whether they be linear oxygen (UEGO), on/off oxygen (lambda) or nitric oxide or hydrocarbon sensors, comprise a diffusion chamber for receiving part of the exhaust gas from the engine; a reference chamber containing a given percentage of oxygen; and an electrolytic (so-called Vs sensing) cell sensitive to oxygen ions and interposed between the diffusion and reference chambers. The electrolytic cell has two electrodes between which, in use, is present a voltage signal related to the difference between the oxygen percentages in the diffusion and reference chambers.
The voltage signal at the terminals of the electrolytic cell is processed to generate an output signal indicating the exhaust gas composition and, hence, the ratio of the mixture supplied to the engine.
To operate correctly, the temperature of such sensors must be maintained about a given optimum temperature value, which depends on the type and physical characteristics of the sensor.
To enable rapid heating of the sensor when cold starting the engine, and to maintain the temperature about the optimum value when the engine is running, each sensor has a respective heater (representable schematically by an electric resistor) current driven by a control device. Heater control devices provide for two functions: regulating the current supplied to the heater (to control the temperature of the sensor) and diagnosing the efficiency of the heater, to prevent any deterioration of the heater resulting in failure to maintain the temperature of the sensor about the optimum value, and the generation of spurious exhaust gas composition signals.
To control the temperature of the sensor, known control devices exploit the relationship between the temperature of the sensor and the internal resistance of the electrolytic cell. More specifically, known devices determine the differential voltage at the terminals of the electrolytic cell before and after supplying a reference current to the cell, and calculate the internal resistance by dividing the difference between the two differential voltages by the reference current. The calculated internal resistance value is then converted into the current temperature of the sensor using a memorized conversion table, and the current temperature is used in a feedback circuit for regulating the current supplied to the heater according to the difference between the current and optimum temperatures.
The heater is diagnosed by measuring the voltage drop at the terminals of a measuring resistor connected in series with the heater, i.e. by determining the current through the heater. More specifically, the heater is considered inefficient when the measured current values fail to fall within the efficiency range specified by the sensor manufacturer.
A major drawback of control devices of the type described above lies in the degree of accuracy with which the internal resistance of the electrolytic cell is measured.
That is, the internal resistance of the cell is measured applying Ohm's law as described above, regardless of the current state of the cell, i.e. regardless of the oxygen percentage of the gases in the diffusion chamber. Whereas, in actual fact, tests have shown the above method of determining internal resistance to result in fairly serious errors, on account of the effect on internal resistance of variations in the oxygen percentage in the diffusion chamber, and therefore in the ratio of the mixture supplied to the engine.
As a result, the current sensor temperature value indicated by known control devices differs significantly from the actual value, thus resulting in feedback circuit errors and possibly also, among other things, in impaired diagnosis.